Fluorosilicone rubber composition

ABSTRACT

The fluorine-substituted organopolysiloxane as the principal ingredient of the inventive silicone rubber composition has vinyl groups to serve as the cross-linking points at the molecular chain terminals while at least the penultimate siloxane unit or the silicon atom adjacent to the terminal silicon atom has no fluorine-substituted hydrocarbon group bonded to the silicon atom. Being freed from the steric hindrance due to the bulky fluorine-substituted hydrocarbon group bonded to the adjacent silicon atom, the vinyl group bonded to the terminal silicon atom can pertain to the cross-linking reaction much easier than otherwise so that the silicone rubber composition of the invention containing an organic peroxide or a combination of an organohydrogenpolysiloxane and a platinum catalyst as a curing agent can be rapidly cured to give a cured silicone rubber article having excellent resistance against oils and organic solvents as well as mechanical properties.

BACKGROUND OF THE INVENTION

This is a continuation-in-part application from a copending U.S. patentapplication Ser. No. 07/157,124 filed Feb. 10, 1988, now abandoned,which is a continuation application of a now abandoned U.S. patentapplication Ser. No. 07/003,946 filed Jan. 16, 1987.

The present invention relates to a curable fluorosilicone compositionor, more particularly, to a fluorosilicone composition having goodcurability and useful as a molding compound for injection molding,gel-like potting material or adhesive or capable of giving a curedsilicone rubber having excellent resistance against oils and organicsolvents, restorability from compression and mechanical strengths so asto be useful as a material for shaping diaphragms and oil seals as apart of, for example, equipments for transportation.

As is well known, fluorosilicone rubbers or, namely, silicone rubbers ofwhich the organopolysiloxane molecules have siliconbonded hydrocarbongroups at least partly substituted by fluorine atoms for the hydrogenatoms, such as 3,3,3-trifluoropropyl group, have excellentcharacteristics such as heat resistance, cold resistance, resistanceagainst oils and solvents, restorability from compression and so on incomparison with other conventional nonfluorine silicone rubbers so thatthey are widely used as a material for shaping various parts oftransportation equipments such as automobiles and aircrafts and variousparts of machines used in petroleum industry. See, for example, U.S.Pat. Nos. 2,979,519 and 3,179,610 teaching a silicone rubber compositionof which the organopolysiloxane molecules have perfluorinatedhydrocarbon groups and which are capable of giving cured siliconerubbers having excellent resistance against hydrocarbon solvents. Theseprior art fluorosilicone rubber compositions are, however, defective inrespect of the low curability that the composition cannot be cured at arelatively low temperature such as room temperature and in respect ofthe poor mechanical properties of the rubber products obtained by curingthe composition even when the composition has been fully cured.

It is usual that organopolysiloxane gums as a principal ingredient of asilicone rubber composition are prepared by the ring openingpolymerization of a cyclic diorganosiloxane oligomer using an alkalicatalyst in the presence of a chain terminal-forming agent such as ahexaorganodisiloxane. It is sometimes desirable to use1,1,3,3-tetramethyl-1,3-divinyl disiloxane as teh chain terminal-formingagent so that a vinyl gruop is introduced into each terminal of thediorganopolysiloxane molecules to serve as a crosslinking point. When afluorine-substituted cyclic diorganosiloxane oligomer is subjected tothe ring-opening polymerization in the above described manner using analkali catalyst in the presence of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane, however, the vinyl groups can hardly be introduced into themolecular chain terminals due to the very specific polymerizationcharacteristics of the cyclic oligomer. Therefore, vinyl groups to serveas the crosslinking points are usually introduced into the molecules ofthe fluorine-substituted organopolysiloxane gum by the copolymerizationof 1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane and1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane.

A problem in the thus prepared fluorine-substituted organopolysiloxanegum as the principal ingredient of a fluorosilicone rubber compositionis the relatively low velocity of the crosslink-forming reaction of thevinyl groups with the free radicals produced from an organic peroxide orof the vinyl groups by the hydrosilation reaction with silicon-bondedhydrogen atoms in an organohydrogenpolysiloxane admixed as acrosslinking agent in the presence of a platinum catalyst presumably dueto the three-dimensional bulkiness of the trifluoropropyl groups.Another problem in the conventional fluorosilicone rubbers is therelatively low mechanical strengths thereof in comparison with ordinaryorganic rubbers including fluorocarbon rubbers.

The reason for the above mentioned defects in the conventionalfluorosilicone rubbers is presumably that when the fluorinated alkylgroups having bulkiness are present in the vicinity of thesilicon-bonded vinyl groups, which should provide the crosslinkingpoints, the vinyl groups can pertain to the crosslinking reaction withthe free radicals produced from an organic peroxide admixed as a curingagent or silicon-bonded hydrogen atoms with which the vinyl groups enterthe hydrosilation reaction to form crosslinks only with a greatlydecreased velocity of the crosslinking reaction consequently not toimpart the cured silicone rubber composition with full mechanicalstrengths.

Several attempts and proposals have of course been made in the prior artto solve the above mentioned problem. For example, U.S. Pat. Nos.4,032,502 and 4,029,629 teach that the curing velocity of afluorosilicone rubber composition can be controlled by using anorganohydrogenpolysiloxane of which the siliconbonded hydrogen atoms areintroduced into the polysiloxane molecules in the form of amonofunctional siloxane units of the formula HR₂ SiO_(1/2), R being amonovalent hydrocarbon group. Needless to say, the applicability of thismethod is limited to the compositions curable by the mechanism ofhydrosilation and no solution of the problem is provided for thefluorosilicone rubber compositions of the type curable with an organicperoxide as a curing agent.

SUMMARY OF THE INVENTION

Thus, the curable fluorine-substituted organopolysiloxane composition ofthe present invention, developed to overcome the above describedproblems and disadvantages in the prior art compositions, comprises, inadmixture:

(A) a polydiorganosiloxane having a viscosity of at least 400 centipoiseat 25° C. and a molecular structure represented by the general formula

    X.sup.1 --O--(--SiR.sub.2 --O--).sub.n --(SiRR.sub.f --O--).sub.m --X.sup.2,(I)

or

    [X.sup.1 --O--(--SiR.sub.2 --O--).sub.n --(--SiRR.sub.f --O--).sub.m --].sub.p SiR.sub.4-p,                                    (II)

in which R is a monovalent hydrocarbon group selected from the classconsisting of methyl, vinyl and phenyl groups, R_(f) is a fluorinatedalkyl group having 3 to 10 carbon atoms, n is zero or a positiveinteger, m is a positive integer with the proviso that m:(m+n) is in therange from 0.005 to 1, p is 2, 3 or 4, X¹ is a vinyl dimethylsiloxy-substituted silyl group represented by the general formula

    Vi--SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.r.spsb.-,

in which Vi is a vinyl group, Me is a methyl group and r is a positiveinteger in the range from 1 to 30, and X² is H or X¹ ;

(B) a curing agent in an amount sufficient to effect curing of thecomponent (A); and

(C) optionally, a finely divided silica filler having a specific surfacearea of at least 50 m² /g.

In particular, the curing agent as the component (B) is (B-1) an organicperoxide in an amount in the range from 0.1 to 10 parts by weight per100 parts by weight of the component (A) or (B-2) a combination of(B-2a) a platinum compound in an amount in the range from 0.1 to 200 ppmby weight as platinum based on the component (A) and (B-2b)anorganohydrogenpolysiloxane having at least three hydrogen atoms directlybonded to the silicon atoms in a molecule in an amount sufficient toprovide from 0.5 to 4.0 moles of the silicon-bonded hydrogen atoms permole of the vinyl groups in the component (A).

When the composition is admixed with the silica filler as the component(C), the amount of the silica filler should be 100 parts by weight orsmaller per 100 parts by weight of the component (A).

BRIEF DESCRIPTION OF THE DRAWING

The figures graphically illustrate the curing characteristics of thefluorosilicone rubber compositions prepared in the examples andcomparative examples, of which

FIG. 1 is for Examples 1 and Comparative Example 1 and

FIG. 2 is a for Example 2 and Comparative Examples 2 to 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is understood fromthe above given summary of the invention, the most characteristicfeature of the inventive fluorosilicone rubber composition consists inthe specific molecular structure of the fluorine-substituted linear orbranched-chain polydiorganosiloxane as the component (A), in which oneof the molecular chain terminals is blocked with a vinyl-containing endgroup or, in particular, with a dimethyl (vinyl dimethyl siloxy) silylgroup of the formula ViMe₂ Si--O--SiMe₂ --. While the vinyl group at themolecular chain end serves as a crosslinking point, the reactivitythereof for the crosslink formation is not affected by thefluorine-containing groups because the vinyl-containing terminal groupis necessarily separated form the siloxane units havingfluorine-substituted groups with at least one dimethylsiloxane unitintervening therebetween so that the curing characteristic of thecomposition is greatly inmproved in comparison with conventionalfluorosilicone rubber compositions.

The component (A) in the inventive fluorosilicone rubber composition isa fluorine-substituted polydiorganosiloxane represented by the generalformula (I) or (II) given above. In the formula (I), one of themolecular chain ends may be blocked with a silanolic hydroxy group whilethe other molecular chain end is necessarily blocked with the groupdenoted by X¹ which is represented by the general formula Vi--SiMe₂--(--O--SiMe₂ --)_(r).spsb.-, in which Vi is a vinyl group, Me is amethyl group and the subscript r is a positive integer in the range from1 to 30. It is of course optional that two differentpolydiorganosiloxanes represented by the general formulas (I) and (II),respectively, are used in combination.

The group denoted by R in the general formulas (I) and (II) is a methyl,vinyl or phenyl group while the group denoted by R_(f) is a fluorinatedalkyl group. Examples of the group denoted by R_(f) include3,3,3-trifluoropropyl, 2-(perfluorobutyl)ethyl and 2-perfluooctyl)ethylgroups.

The subscript n in the general formulas (I) and (II) is zero or apositive integer and the subscript m is a positive integer. The valuesof these subscripts are not particularly limitative provided that theorganopolysiloxane has a viscosity of at least 400 centipoise at 25° C.The value of m:(m+n) should preferably be in the range from 0.005 to 1so that the organopolysiloxane contains the fluorinated group-containingsiloxane units at least in a substantial molar proportion.

The fluorine-substituted polydiorganosiloxane represented by the generalformula (I) can be prepared, for example, by the following method. Thus,(i) 1 mole of vinyl dimethyl silanol of the formula ViMe₂ SiOH isreacted with 1 to 10 moles of metallic lithium at a temperature in therange from -10 to +50° C. for a length of time of 0.5 to 24 hours togive vinyl dimethyl siloxy lithium of the formula ViMe₂ SiOLi, in whichMe and Vi are methyl and vinyl groups, respectively, and then (ii) 1mole of this siloxy lithium compound is reacted with 0.333 to 10 molesor, preferably, 1 to 10 moles of hexamethyl cyclotrisiloxane of theformula (Me₂ SiO)₃ at a temperature in the range from -10° C. to +50° C.for a length of time of 0.5 to 24 hours to give a lithium-terminatedorganosiloxane oligomer of the formula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O--).sub.4 --Li          (III)

in which r is a positive integer of 1 to 30 Thereafter, (iii) a ringopening polymerization reaction of 5 to 50 moles of1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane ofthe formula [Si(CH₃)(CH₂ CH₂ CF₃)--O]₃ is performed in the presence of0.01 mole of the above obtained organosiloxane oligomer of the formula(III) as a polymerization catalyst at a temperature in the range from50° C. to 180° C. for a length of time of 0.5 to 24 hours to give alithium-terminated polydiorganosiloxane of the formula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O--).sub.4 --(--SiMeP.sub.f --O--).sub.m -Li,                                                      (IV)

in which Me is a methyl group, P_(f) is a 3,3,3trifluoropropyl group anm and r each have the same meaning as defined above, followed by (iv)inactivation of the lithium-blocked terminal by the neutralizationtreatment with a mixture of a chlorosilane and an organosiloxane and/ora weak acid into the polydiorganosiloxane of the general formula (I)above.

The first and the second steps of the above described process, i.e. thereaction of metallic lithium with vinyl dimethyl silanol andoligomerization of the hexamethyl cyclotrisiloxane, should preferably beperformed in a reaction medium of a polar organic solvent such astetrahydrofuran with an object to enhance the reactivity of the solidlithium with the silanol compound and to increase the solubility of thesiloxy lithium compound as well as to facilitate removal of theunreacted metallic lithium by filtration. Although the reaction mixtureobtained in the above mentioned first step (i) and freed from theunreacted metallic lithium can be used as such in the subsequent step ofthe oligomerization of hexamethyl cyclotrisiloxane, it is preferablethat the reaction mixture obtained in the second step (ii) is freed fromthe organic solvent as completely as possible before it is used in thethird step of the ring-opening polymerization of1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane sincethe polar organic solvent such as tetrahydrofuran may promotere-equilibration reaction of the once formed polydiorganosiloxane.

The chlorosilane-organosiloxane mixture used in the above mentionedinactivation treatment of the lithium-terminated polydiorganosiloxane ofthe formula (IV) is preferably a mixture of 1 mole of a methylchlorosilane of the formula Me_(a) SiCl_(4-a), a being zero, 1,2 or 3,and 1 to 10 moles of hexamethyl disilazane of the formula Me₃Si--NH--SiMe₃ and the mixture is used in such an amount that from 1 to50 moles of the silicon-bonded chlorine atoms are provided per mole oflithium in the siloxy lithium compound. The weak acid as theneutralizing agent may be acetic acid used in an amount also in therange from 1 to 50 moles per mole of lithium.

Although a part of the methyl groups in the general formula (I) may bereplaced with other monovalent hydrocarbon groups, it is preferable thatall of the organic groups bonded to the silicon atoms other than theterminal vinyl groups and the fluorinated groups R_(f) are all methylgroups. In particular, the methyl groups bonded to the terminal andpenultimate silicon atoms should not be replaced with other more bulkyhydrocarbon groups in order not to cause steric hindrance to theterminal vinyl group in the crosslinking reaction by the attack of thefree radicals produced from an organic peroxide added to the compositionas the curing agent or of the silicon-bonded hydrogen atoms of anorganohydrogenpolysiloxane atoms to effect the hydrosilation reaction.

The subscript r in the general formula giving the terminal group X¹should be a positive integer not exceeding 30. When r is larger than 30,the velocity of the polymerization reaction int he third step (iii)described above would be greatly decreased because of the decrease inthe solubility of the siloxane oligomer used as the catalyst in the1,3,5-trimethyl-1,3,5-tris (3,3,3-trifluoropropyl) cyclotrisiloxane.Preferably, the subscript should be an integer of 3 to 12. The subscriptp in the general formula (II) is 2, 3 or 4 without particularlimitation.

An alternative method for the preparation of the organopolysiloxanerepresented by the general formula (I) as the component (A) is asfollows. Thus, a trimetyl trifluoroalkyl cyclotrisiloxane or atetramethyl tetra(fluoroalkyl) cyclotetrasiloxane is admixed withhexamethyl cyclotrisiloxane or octamethyl cyclotetrasiloxane in asuitable proportion depending on the desired product together with acatalytic amount of a basic compound, e.g., potassium hydroxide, cesiumhydroxide, tetramethyl phosphonium hydroxide and tetramethyl ammoniumhydroxide as well as siliconates thereof, and water in an amountsufficient to serve as a chain-end stopper and the mixture is heatedunder agitation at a temperature of 80° C. to 200° C. or, preferably,100° C. to 150° C. to give a hydroxy-terminated diorganopolysiloxane ofthe general formula

    OH--(--pi SiMe.sub.2 --O--).sub.n --(--SiMeR.sub.f --O--).sub.m --H,(V)

in which each symbol has the same meaning as defined above.

Separately, vinyl dimethyl chlorosilane and hexamethyl cyclotrisiloxaneare reacted at room temperature in the presence of a catalyst such ashexamethyl phosphoric triamide to give a vinyl and chlorine-terminateddimethylpolysiloxane of the formula

    Vi--SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.r --Cl,          (VI)

in which each symbol has the same meaning as defined above. The value ofr can be adequately controlled by suitably selecting the molarproportion of the vinyl dimethyl chlorosilane and hexamethylcyclotrisiloxane.

In the next place, the hydroxy-terminated polysiloxane of the generalformula (V) and the vinyl-and chlorine-terminated polysiloxane of thegeneral formula (VI) are subjected to a dehydrochlorination reaction inthe presence of a hydrogen chloride acceptor such as tertiary amines andorganosiloxane compounds to give a vinyl-terminated polydiorganosiloxaneof the formula ##STR1##

A still alternative method is as follows. Thus, a trimethyltri(fluoroalkyl) cyclotrisiloxane such as1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane isdissolved in a polar organic solvent such as acetonirtile,tetrahydrofuran, ethyl acetate, dimethyl sulfoxide and the like and keptstanding in the presence of a pentavalent coordination compound ofsilicon such as those disclosed in Japanese Patent Publication 45-1070as a catalyst at a temperature of 0° C. to 50° C. or, preferably, 10° C.to 30° C. for 10 to 20 hours with further addition of a small amount ofwater to provide the hydroxy terminal groups so as to give ahydroxy-terminated diorganopolysiloxane of the general formula

    HO--(--SiMeR.sub.f --O--).sub.m --H.                       (VIII)

This hydroxy-terminated diorganopolysiloxane is then subjected to adehydrochlorination reaction with the vinyl-and chlorine-terminatedorganopolysiloxane of the general formula (VI) given above to give thedesired polydiorganosiloxane of the general formula ##STR2##

Further, the vinyl-and chlorine-terminated organopolysiloxane of thegeneral formula (VI) given above is added dropwise to a non-polarorganic solvent such as toluene, xylene, n-hexane and the like in anamount of 5 to 10 times thereof containing sodium hydrogen carbonatedispersed therein in an amount of 1.5 to 2.0 times by moles of thechlorine contained int eh organopolysiloxane so that the chlorine atomat one of the molecular chain ends is converted into a hydroxy group togive a vinyl-and hydroxy-terminated polydiorganosiloxane. When thering-opening polymerization reaction of a trimethyl tris(fluoroalkyl)cyclotrisiloxane in a polar organic solvent as catalyzed by apentavalent coordination compound of silicon is performed in thepresence of such a vinyl-and hydroxy-terminated polydiorganosiloxane,the resultant product is a desired polyorganosiloxane of the generalformula

    Vi--SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.4 --(--O--SiMeR.sub.f --).sub.m --OH.                                                     (X)

The above described vinyl-and hydroxy-terminated polydiorganosiloxane ofthe general formula (X) can be used as an intermediate for thepreparation of the polyorganosiloxane represented by the general formula(II) as the component (A) by reacting the same with an organosiloxanecompound represented by the general formula

    Y.sub.p SiR.sub.4-p                                        , (XI)

in which R and p each have the same meaning as defined before and Y is achlorine atom or an amino group substituted with methyl, ethyl or propylgroups to give a polyorganosiloxane of the general formula

    [ViSiMe.sub.2 --O--(--SiMe.sub.2 --O--).sub.r --(--SiRR.sub.f --O--).sub.m --].sub.p SiR.sub.r-p,                                    (XII)

in which each symbol has the same meaning as defined before.

A typical example of the fluorine-substituted polydiorganosiloxanehaving vinyl groups at both molecular chain ends as the component (A)expressed by the following structural formula: ##STR3## in which n and mare each a positive integer satisfying the relationship that n+m is inthe range from 30 to 3000 and m:(m+n) is in the range from 0.005 to1.00, the other symbols each having the same meaning as defined before.

The second essential ingredient in the inventive composition, i.e.component (B), is a curing agent of the above described component (A).Since the polyorganosiloxane as the component (A) has vinyl groups asthe crosslinking points in the molecular structure, know methods ofcuring an organopolysiloxane of this type can be used also in this case,Namely, the polyorganosiloxane can be cured by compounding with anorganic peroxide and heating the composition. Alternatively, thepolyorganosiloxane can be cured by compounding with anorganohydrogenpolysiloxane together with a catalytic amount of aplatinum compound as a catalyst to effect the platinum-catalyzedaddition reaction or so-called hydrosilation reaction between thesilicon-bonded vinyl groups and the silicon-bonded hydrogen atoms, ifnecessary, by heating. Either of these two methods can provide a curedsilicone rubber at a remarkably increased curing velocity and/or providea cured silicone rubber article having greatly improved mechanicalproperties as compared with conventional fluorine-containing siliconerubber compositions.

Examples of organic peroxide suitable as a curing agent of the component(A) in the inventive composition include various kinds of conventionallyused ones for curing of silicone rubber compositions such as diacylperoxides, e.g., benzoyl peroxide and 2,4-dichlorobenzoyl peroxide,alkyl peroxides, e.g., di-tert-butyl peroxides and2,5-dimethyl-2,5-di(tert-butyl peroxy) hexane, and alkaryl peroxides,e.g., dicumyl peroxide. The amount of the organic peroxide as the curingagent should be in the range from 0.1 to 10 parts by weight or,preferably, from 0.2 to 6 parts by weight per 100 parts by weight of thefluorine-substituted polydiorganosiloxane as the component (A) in orderto fully cure the composition.

The composition of the invention compounded of the component (A) and anorganic peroxide as the component (B) is heated at a temperature in therange from 100° to 200° C. or, preferably, from 120° to 180° C. in orderto efficiently cure the composition. Oxygen in the atmospheric air mayhave some retarding influences on the curing velocity so that the curingreaction of the composition is performed preferably under a hermeticallysealed condition or by compression molding to prepare a shaped siliconerubber article.

When curing of the component (A) is performed by the hydrosilationreaction, the curing agent is a combination of anorganohydrogenpolysiloxane and a platinum catalyst. The platinumcatalyst to be added to the inventive composition in combination with anorganohydrogenpolysiloxane may be any of known ones including platinumblack, chloroplatinic acid optionally modified with an alcohol,complexes of chloroplatinic acid with a vinyl-containingorganopolysiloxane, olefin or aldehyde and the like. The amount of theplatinum catalyst to be added to the inventive composition should be inthe range from 0.1 to 200 ppm by weight as platinum based on the amountof the fluorine-substituted polyorganosiloxane as the component (A).

The organohydrogenpolysiloxane used as a crosslinking agent of thecomponent (A) should have at least three silicon-bonded hydrogen atoms,i.e. hydrogen atoms directly bonded to the silicon atoms, in a moleculein order to effectively form crosslinks while the molecularconfiguration thereof is not particularly limitative includingstraightly linear, branched chain-like and cyclic ones. The amount ofthe organohydrogenpolysiloxane in the inventive fluorosilicone rubbercomposition should be sufficient to provide form 0.5 to 4.0 moles of thesilicon-bonded hydrogen atoms per mole of the vinyl groups in thefluorine-substituted polyorganosiloxane as the component (A).

Examples of the organohydrogenpolysiloxane suitable as the crosslinkingagent include those expressed by the following structural formulas:##STR4## in which u is a positive integer and v and w are each zero or apositive integer, as well as organohydrogenpolysiloxanes having aresinous structure as composed of the monofunctional siloxane units ofthe formula HSiMe₂ O₀.5 and tetrafunctional siloxane units of theformula SiO₂, in which the molar ratio of (H+Me)/Si is 1.0 to 2.7, andthose composed of the monofunctional siloxane units of the formulaHSiMe₂ O₀.5, difunctional siloxane units of the formula SiMe₂ O and/orSiMeR_(f) O and tetrafunctional siloxane units of the formula SiO₂, inwhich the molar ratio of (H+Me)/Si is 1.2 to 2.7.

The component (C), which is an optional ingredient compounded in theinventive composition according to need, is a finely divided siliceousfiller which may be any of known ones conventionally used in siliconerubber compositions as a reinforcing filler including silica hydrogelsand silica aerogels, e.g., fumed silica and precipitated silica. It isessential that the siliceous filler should have a specific surface areaof at least 50 m² /g as determined by the BET method is order that thefiller may exhibit a full reinforcing effect. The amount of the finelydivided siliceous filler as the component (C) in the inventivefluorosilicone rubber composition should be int eh range from 5 to 100parts by weight per 100 parts by weight of the fluorine-substitutedpolydiorganosiloxane as the component (A). When the amount of the filleris too small, the reinforcing effect to be obtained with the fillerwould be insufficient so that the composition cannot give a curedsilicone rubber article having high mechanical properties as desired.When the amount of the siliceous filler is too large, on the other hand,difficulties are encountered in the compounding work of the componentsand, even if a composition could be prepared somehow, the composition istoo stiff and poorly moldable and the cured silicone rubber thereof alsomay have poor mechanical properties.

The fluorosilicone rubber composition of the present invention can beprepared by uniformly mixing the above described components (A), (B)and, optionally, (C) each in a calculated and weighed amount using aconventional mixing machine such as gate mixers, butterfly mixers,Banbury mixers, kneaders, intermixers, two-roller mills and the like. Itis preferable in this mixing work, when the component (C) is used, thatthe components (A) and (C) alone are first mixed together and themixture of these components is subjected to a heat treatment prior toadmixture with the component (B). It is of course optional that thefluorosilicone rubber composition of the invention is admixed accordingto need with various kinds of known additives including non-reinforcinginorganic fillers, e.g. diatomaceous earth, finely pulverized quartzpowder, powder of fused quartz glass, clay, aluminum oxide, calciumcarbonate and talc, dispersion aids, e.g., low-molecularorganopolysiloxane esters and silanol compounds, heat-resistanceimprovers. e.g., iron oxide and ceric oxide,electroconductivity-imparting agents, e.g., carbon black, and so on eachin a limited amount.

When the thus prepared composition of the invention has a flowableconsistency, it can be used as a potting agent, adhesives or sealingagent which is requires to exhibit excellent resistance against oils.When the composition has a consistency as a rubber compound and used forpreparing rubber articles, the fluorosilicone rubber composition of theinvention prepared int eh above described manner is then shaped into adesired form and subjected to curing into a cured silicone rubberarticle. THe method for shaping the composition is not particularlylimitative and may be the same as for conventional silicone rubbercompositions including compression molding, transfer molding andinjection molding using a metal mold and calendering and extrusionmolding for continuous-length forms depending on the particular form ofthe desired products. The thus shaped composition is subjected to curingby heating at 100° to 400° C. for a length of time of 30 seconds to 1hour under normal pressure or under increased pressure and then tosecondary curing according to need at 150° to 250° C. for 1 to 24 hoursinto a fully cured silicone rubber article having excellent mechanicalstrengths. The cured fluorosilicone rubber obtained in this manner hasexcellent resistance against oils and organic solvents, restorabilityfrom compression and mechanical strengths so that the fluorosiliconerubber composition fothe invention is useful as a material of diaphragmsand oil seal as a part of transportation equipments.

In the following, the fluorosilicone rubber composition of the presentinvention is described in more detail by way of examples in which theterm of "parts" always refers to "parts by weight".

PREPARATION 1 Preparation of the polymerixation catalyst

Into a flash of 200 ml capacity were introduced 10.2 g (0.1 mole) ofvinyl dimethyl silanol and 100 ml of anhydrous tetrahydrofuran and then1.0 g (0.14 mole) of metallic lithium in fine chips was added in threeportions to the mixture in the flask under a stream of nitrogen. Themixture was agitated at room temperature for 10 hours to effect thereaction. Filtration of the reaction mixture to remove the unreactedmetallic lithium gave a tetrahydrofuran solution of vinyl dimethylsiloxy lithium of the formula ViMe₂ SiOLi.

In the next place, a solution of 44.4 g (0.2 mole) of hexamethylcyclotrisiloxane dissolved in 35 ml of tetrahydrofuran was addeddropwise to the above obtained tetrahydrofuran solution of vinyldimethyl siloxy lithium and the reaction mixture was agitated for 24hours at room temperature followed by distillation at 50° C. under areduced pressure of 3 mmHg to remove tetrahydrofuran leaving 54 g of aclear oily material which could be identified by elementary analysis tobe an organosiloxane oligomer expressed by the formula ViMe₂Si--O--(--SiMe₂ --O--)₆ --Li.

PREPARATION 2 Preparation of the fluorosilicone gum I

In a flask with a separable cover of 2 liter capacity were taken 1560 g(3.33 moles) of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane which was heated at 150°C. under an atmosphere ofnitrogen gas and then 1.38 g (0.0025 mole) of the organosiloxaneoligomer obtained in Preparation 1 were added thereto under agitation.The mixture in the flask was further heated at 150° C. for 4 hours toeffect the polymerization reaction and the thus obtainedfluorine-substituted polydiorganosiloxane gum was transferred to akneader and admixed there with 10 g (0.031 mole) of a 1.0:1.1 by molesmixture of vinyl dimetyl chlorosilane and vinyl dimethyl silazane as aneutralizing agent. The mixture was agitated first at room temperaturefor 1 hour and then at 150° C. for 1 hour to give a fluorine-substitutedpolydiorganosiloxane gum expressed by the formula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O--).sub.6 --(--SiMeP.sub.f --O--).sub.3996 --SiMe.sub.2 Vi,

in which P_(f) is a 3,3,3-trifluoropropyl group. This product isreferred to as the Silicone Gum I hereinbelow.

PREPARATION 3 Preparation of the fluorosilicone gum II

Polymerization of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane in the presence of the organosiloxy lithium prepared inPreparation 1 was performed in the same manner as in Preparation 2 andthe thus obtained lithium-terminated polydiorganosiloxane gum wassubjected to a neutralization treatment by adding, in place of themixture of vinyl dimethyl chlorosilane and vinyl dimethyl silazane, 1.2g(0.02 mole) of acetic acid in a kneader and the mixture was agitatedfirst at room temperature for 1 hour and then at 150° C. for 1 hour togive a fluorine-substituted polydiorganosiloxane gum expressed by theformula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O--).sub.6 --(--SiMeP.sub.f --O--).sub.3996 --H,

which is referred to as the Silicone Gum II hereinbelow.

PREPARATION 4 Preparation of the fluorosilicone gum III

An oily oligomeric organosiloxy lithium of the formula

    Me.sub.e Si--O--(--SiMe.sub.2 --O--).sub.6 --Li

was prepared in the same manner as in Preparation 1 exceptingreplacement of the vinyl dimethyl silanol with 8.0 g (0.1 mole) oftrimethyl silanol and the oragnosiloxy lithium compound was used in thepolymerization of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane in the same manner as in Preparation 2 to give alithium-terminated fluorine substituted polydiorganosiloxane gum whichwas neutralized in the same manner as in Preparation 3 using acetic acidas the neutralizing agent to give a fluorine-substitutedpolydiorganosiloxane gum terminated at one molecular chain end with atrimethyl siloxy group and at the other end with a silanol group asexpressed by the formula

    Me.sub.3 Si--O--(--SiMe.sub.2 --O--).sub.6 --(--SiMeP.sub.f --O--).sub.3996 --H,

which is referred to as the Silicone Gum III hereinbelow.

PREPARATION 5-A Preparation of the fluorosilicone gum IV-A

Copolymerization of 1560 g of1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane and1.29 g of 1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane was performedby using the oily organosiloxy lithium prepared in Preparation 4 as thecatalyst and the thus obtained lithium-terminated polydiorganosiloxanegum was neutralized in the same manner as in Preparation 3 with aceticacid as the neutralizing agent to give a fluorine-substitutedpolydiorganosiloxane gum, in which 0.15% by moles of the siliconbondedorganic groups were vinyl groups, terminated at one of the molecularchain ends with a trimethyl silyl group and at the other molecular chainend with a silanol group as expressed by the formula

    Me.sub.3 Si--O--(--SiMe.sub.2 --O--).sub.6 --(--SiMeP.sub.f --O--).sub.3996 --(--SiViMe--O--).sub.6 --H,

which is referred to as the Silicone Gum IV-A hereinbelow.

PREPARATION 5-B Preparation of the fluorosilicone gum IV-B

The procedure for the preparation for a fluorine-substitutedpolydiorganosiloxane gum expressed by the structural formula

    Me.sub.3 Si--O--(--SiMe.sub.2 --O--).sub.6 --(--SiMeP.sub.f --O--).sub.3996 --(--SiViMe--O--).sub.6 --H,

which is referred to as the Silicone Gum IV-B hereinbelow, wassubstantially the same as in the above described Preparation 5-Aexcepting replacement of the acetic acid used as the neutrailizing agentwith a mixture of vinyl dimethyl chlorosilane and vinyl dimethylsilazane as used in Preparation 2.

PREPARATION 6 Preparation of the fluorosilicone gum V

A polymerization catalyst was prepared in the same manner as inPreparation 1 except that 44.4 g (0.2 mole) of hexamethylcyclotrisiloxane were replaced with 93.6 g (0.2 mole) of1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane. Thispolymerization catalyst was used in the preparation of a fluorosiliconegum, which is referred to as the Silicone Gum V hereinbelow, in the samemanner as in Preparation 2. Different from the Silicone Gum I preparedin Preparation 2, the Silicone Gum V had a molecular structure in whichthe vinyl dimethyl silyl group at the molecular chain terminal wasbonded directly to a siloxane unit having a 3,3,3-trifluoropropyl groupbonded to the silicon atom.

PREPARATION 7 Preparation of the fluorosilicone oils VI and VII

Into a flask of 5 liter capacity were introduced 3900 g of1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane, 1.8g of water, 1000 g of acetonitrile and 0.3 g of phenyl biscatecholbenzyl trimethyl ammonium silicate (BTMAS) and the mixture was agitatedfor 8 hours at a temperature in the range from 0° C. to 25° C.Thereafter, 103 g of 1-vinyl-7-chloro-1,1,3,3,5,5,7,7-octamethyltetrasiloxane and 85 g of 1,3-divinyl-1,1,3,3-tetramethyl disilazanewere added to the mixture which was further agitated followed byfiltration to remove the salt precipitated in the mixture. The filtratewas subjected to stripping of volatile matter at 100° C. under apressure of 5 mmHg to give an oily product having a viscosity of 40,000centipoise at 25° C. and expressed by the structural formula

    Vi--SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.3 --(--O--SiMeP.sub.f --).sub.250 --(--O--SiMe.sub.2 --).sub.3 --O--SiMe.sub.2 Vi,

which is referred to as the Silicone Oil VI hereinbelow.

Separately, another run of the same procedure as above was undertakenexcept that the neutralization step was performed by using vinyldimethyl chlorosilane instead of1-vinyl-7-chloro-1,1,3,3,5,5,7,7-octamethyl tetrasiloxane also to givean oily product expressed by the structural formula

    Vi--SiMe.sub.2 --(--O--SiMeP.sub.f --).sub.250 --(--O--SiMe.sub.2 Vi,

which is referred to as the Silicone Oil VII hereinbelow.

PREPARATION 8 Preparation of the fluorosilicone oils VIII and IX

Into a flask of 5 liter capacity were introduced 450 g of anα,ω-dihydroxydimethyl polysiloxane of the formula

    HO--SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.10 --O--SiMe.sub.2 OH,

2730 g of 1,3,5-trimethyl-1,3,5-tris(3,3,3(trifluoropropyl)cyclotrisiloxane, 600 g of acetonitrile and 0.3 g of phenyl biscatecholbenzyl trimethyl ammonium silicate (BTMAS) and the polymerizationreaction was performed in the same manner as in Preparation 7 above. Thereaction mixture after completion of the polymerization reaction wasneutralized by using 1-vinyl-7-chloro-1,1,3,3,-5,5,7,7-octamethyltetrasiloxane and 1,3-divinyl-1,1,3,3-tetramethyl disilazane to give anoily product having a viscosity of 700 centipoise at 25° C. andexpressed by the structural formula ##STR5## which is referred to as theSilicone Oil VIII herebelow.

Separately, another run of the same procedure as above was undertakenexcept that the neutralization step was performed by using vinyldimethyl chlorosilane instead of1-vinyl-7-chloro-1,1,-3,3,5,5,7,7-octamethyl tetrasiloxane also to givean oily product having a viscosity of 700 centipoise at 25° C. andexpressed by the structural formula ##STR6## which is referred to as theSilicone OIl IX hereinbelow.

Preparation 9 Preparation of the fluorosilicone gum X

The procedure for the preparation of a fluorine-substitutedpolydiorganosiloxane gum was substantially the same as in Preparation 3described above except that the1,3,5-trimethyl-1,3,5-tris-(3,3,3-trifluoropropyl) cyclotrisiloxane wasreplaced with a combination of 2.58 g (0.01 mole) of1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane and 1560 g (3.33 moles)of 1,3,5-trimethyl-1,3,5-tris-(3,3,3-trifluoropropyl) cyclotrislioxane.The thus obtained fluorine-substituted polydiorganosiloxane, which isreferred to as the Silicone Gum X hereinbelow, was expressed by thestructural formula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O--).sub.6 --[--(--SiMeP.sub.f --O--).sub.n --(--SiViMe--O--).sub.m --].sub.4000 --H,

in which ma and n are each a positive integer with the proviso ofm:(m+n)=0.003.

Preparation 10 Preparation of the fluorosilicone gum XIII

The procedure for the preparation of a fluorine-substitutedpolydiorganosiloxane gum was substantially the same as in Preparation 4described above except that the1,3,5=trimethyl-1,3,5-tris-(3,3,3-trifluoropropyl) cyclotrisiloxane wasreplaced with a combination of 2.58 g (0.01 pl mole) of1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane and 1560 g (3.33 moles)of 1,3,5-trimethyl-1,3,5-tris-(3,3,3-trifluoropropyl) cyclotrisiloxane.The thus obtained fluorine-substituted polydiorganosiloxane, which isreferred to as the Silicone Gum XIII hereinbelow, was expressed by thestructural formula

    ViMe.sub.2 Si--O--(--SiMe.sub.2 --O --).sub.6 --.sub.6 [--(--SiMePf--O--).sub.n --(--SiViMe--O--).sub.m ].sub.4000 --H,

in which m and n are each a positive integer with the proviso ofm:(m+n)=0.003.

EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLES 1 TO 4. Example 1

A fluorosilicone rubber compound was prepared in Example 1 by uniformlyblending 100 parts of the Silicone Gum II prepared in Preparation 3 with28 parts of a fumed silica filler having a specific surface area of 300m² /g and 8.5 parts of an α,ω-dihydroxy poly(methyl3,3,3-trifluoropropyl siloxane) having a viscosity of 80 centistokes at25° C. on a two-roller mill followed by a heat treatment at 160° C. for2 hours.

EXAMPLE 2

Similarly to Example 1 described above, another fluorosilicone rubbercompound was prepared in Example 2 by uniformly blending, on atwo-roller mill, 100 parts of the Silicone Gum I prepared in Preparation2 with 20 parts of a fumed silica filler having a specific surface areaof 130 m² /g and blocked on the surface with trimethyl silyl groupsbonded thereto by a surface treatment.

Comparative Example 1

A silicone rubber compound was prepared in the same formulation as inExample 1 above excepting replacement of the Silicone Gum II with thesame amount of the Silicone Gum IV-A prepared in Preparation 5-A.

Comparative Examples 2 to 4

Silicone rubber compounds were prepared in Comparative Examples 2, 3 and4 each in the same formulation as in Example 2 above exceptingreplacement of The Silicone Gum I with the same amount of the SiliconeGum III, V and IV-B prepared in Preparations, 4, 6 and 5-B,respectively.

Each of the thus prepared fluorosilicone rubber compounds was furtheradmixed with 0.8% by weight (Example 1 and Comparative Example 1) or0.6% by weight (Example 2and Comparative Examples 2 to 4) by weight of2,5-dimethyl-2,5-di(tert-butyl peroxy) hexane as a curing agent andshaped by sheeting into a sheet having a thickness of 2 mm, which waspress-cured by heating at 165° C. for 10 minutes under a compressiveforce of 30 kg/cm² into a sheet of cured rubbery elastomer. The curingcharacteristics of these six rubber compositions are shown in FIGS. 1and 2 as measured by using a disc rheometer at a temperature of 170° C.and ARC at 3° C, of which the curves A and B in FIG. 1 is for Example 1and Comparative Example 1, respectively, and the curves C, F, D and Eare for Example 2 and Comparative Examples 2, 3 and 4, respectively.Practically no curing could be obtained in the rubber compositionprepared in Comparative Example 2 using the Silicone Gum III.

The cured silicone rubber sheets prepared in Examples 1 and 2 andComparative Examples 1 to 3 were each subjected to the measurement ofthe physical properties including specific gravity (d₂₅), hardness, JISA (H), ultimate elongation (E) in %, tensile strength (TS) in kg/cm² andtear strength (T_(RA)) in kg/cm. The results are shown in Table 1 below.Further, the silicone rubber sheets prepared in Example 1 andComparative Example 1 were immersed in the JIS #3 oil at 150° C. for 170hours or in Fuel B at room temperature for 24 hours and the decrements(-) or increments (+) in % of the hardness (ΔH), ultimate elongation(ΔE), tensile strength (ΔTS) and volume (ΔV) were determined. Theresults are also shown in Table 1.

EXAMPLE 3 AND COMPARATIVE EXAMPLE 5

In Example 3, 100 parts of the Silicone Oil VI prepared in Preparation 7as uniformly blended with 25 parts of a finely divided wet-processsilica filler having a specific surface area of 115 m² /g and 2 parts ofhexamethyl disilazane at a temperature of 130 to 150° C. in a kneaderblender taking 6 hours. The blend was further admixed with 1 part of1,3,5,7-tetravinyl-1,3,5,7-tetramethyl cyclotetrasiloxane, 4.0 parts ofan organohydrogenpolysiloxane having a viscosity of 50 centistokes at25° C. and expressed by the structural formula

    (H--SiMe.sub.2 --O--SiMeP.sub.f --O--).sub.3 Si--CH.sub.2 CH.sub.2 CH.sub.2 --Si(--O--SiMeP.sub.f --O--SiMe.sub.2 H).sub.3

and chloroplatinic acid modified with 2-ethyl hexyl alcohol in an amountof 20 ppm by weight as platinum based on the Silicone Oil VI.

In Comparative Example 5, another composition was prepared in the sameformulation as above excepting replacement of the Silicone Oil VI withthe same amount of the Silicone Oil VII.

Each of these compositions having a specific gravity of 1.40 wassubjected to the evaluations of the curing characteristics by thedetermination of the T₉₀ value, i.e. the length of time taken until themoment when the torque of mixing had reached 90% of the ultimate value,on an oscillating disc rheometer at 150° C. Further, each compositionwas shaped into a sheet of 2 mm thickness by compression molding at 170°C. taking 5 minutes followed by a post-curing at 200° C. For 4 hours.The results of measurements were as shown below.

Silicone Oil VI: T₉₀ of 45 seconds; hardness, JIS A, of 72; tensilestrength of 200 kg/cm² ; and ultimate elongation of 80%.

Silicone Oil VII: T₉₀ of 60 seconds; hardness, JIS A, of 70; tensilestrength of 160 kg/cm² ; and ultimate elongation of 65%.

                  TABLE 1                                                         ______________________________________                                        Silicone Gum II      I       IV-A  VI-B  V                                    ______________________________________                                                  d.sub.25                                                                             1.41    1.38  1.41  1.38  1.38                                         H       38     26     38   24    23                                 As cured  E       530    590    460  595   620                                          TS      132    120    95   88    73                                           T.sub.RA                                                                              21     20     14   12    10                                           ΔH                                                                              -4            -4                                            After im- ΔE                                                                             -12           -10                                            mersion   ΔTS                                                                            -22           -18                                            in #3 oil ΔV                                                                              +3            +3                                                      ΔH                                                                              -9           -10                                            After im- ΔE                                                                             -38           -40                                            mersion in                                                                              ΔTS                                                                            -52           -38                                            Fuel B    ΔV                                                                             +19           +20                                            ______________________________________                                    

EXAMPLE 4 AND COMPARATIVE EXAMPLE 6

In Example 4, 100 parts of the Silicone Oil VII prepared in Preparation7 were admixed with 1.5 parts of a methyl hydrogen polysiloxaneexpressed by the structural formula

    H--SiMe.sub.2 --(--O--SiMeH--).sub.4.5 --(--O--SiMe.sub.2

and a chloroplatinic acid-vinyl siloxane complex in an amount of 50 ppmby weight as platinum based on the Silicon Oil VI.

IN Comparative Example 6, another composition was prepared in the sameformulation as above excepting replacement of the Silicone Oil VI withthe same amount of the Silicone Oil VII.

Each of the compositions was subjected to the evaluation of the curingcharacteristics at 150° C. by the determination of the T₁₀ and T₉₀values, i.e. the lengths of time taken until the moment when the torqueof mixing had reached 10% and 90%, respectively, of the ultimate value,on an oscillating disc rheometer. The results were that T₁₀ and T₉₀ forthe Silicone Oil VI were 105 seconds and 180 seconds, respectively, andT₁₀ and T₉₀ for the Silicone Oil VII were 125 seconds and 720 seconds,respectively.

EXAMPLE 5 AND COMPARATIVE EXAMPLE 7

A curable composition was prepared in Example 5 in the same formulationas in Example 1 excepting replacement of the Silicone Gum II with thesame amount of a fluorine-containing diorganopolysiloxane oil, which isreferred to as the Silicone Oil XI herebelow, having a viscosity of25,000 centipoise at 25° C. and expressed by the structural formula

    Vi--SiMe.sub.2 --O--(--SiMe.sub.2 --O--).sub.385 --(--SiMeP.sub.f --O--).sub.165 --SiMe.sub.2 Vi

and also replacing 28 parts of the fumed silica filler with 40 parts ofa finely divided wet-process silica filler having a specific surfacearea of 115 m² /g as combined with 2.0 parts of hexamethyl disilazane.

In Comparative Example 7, another curable composition was prepared inthe same formulation as in Example 5 above excepting replacement of theSilicone Oil XI with another fluorine-containing diorganopolysiloxaneoil, which is referred to as the Silicone Oil XII hereinbelow, having aviscosity of 25,000 centipoise at 25° C. and expressed by the structuralformula ##STR7##

Each of the compositions having a specific gravity of 1.28 was subjectedto shaping and curing in the same manner as in Example 1 to give a curesilicone rubber sheet. The T₉₀ values of the compositions and themechanical properties of the cured rubber sheets were as follows.

Silicone Oil XI: T₉₀ of 40 seconds; hardness, JIS A, of 47; tensilestrength of 85 kg/cm² ; and ultimate elongation of 410%.

Silicone Oil XII: T₉₀ of 55 seconds; hardness, JIS A, of 45; tensilestrength of 65 kg/cm² ; and ultimate elongation of 350%.

EXAMPLE 6

A fluorosilicone rubber compound was prepared by uniformly blending 100parts of the Silicone Gum X prepared in Preparation 9 with 30 parts of afumed silica filler having a specific surface area of 200 m² /g and 6.5parts of an α,ωdihydroxy poly(methyl 3,3,3-trifluoropropyl siloxane)having a viscosity of 80 centistokes at 25° C. on a two-roller millfollowed by a heat treatment at 160° C. for 2 hours.

The thus obtained silicone rubber composition was cured in the samemanner as in Example 1 into a rubber sheet which has properties of: aspecific gravity d₂₅ of 1.43; hardness, JIS A, of 44; ultimateelongation of 425%; tensile strength of 126 kg/cm² ; and tear strengthof 27 kg/cm.

COMPARATIVE EXAMPLE 8

A silicone rubber compound was prepared in the same formulation as inExample 6 described above excepting replacement of the Silicone Gum Xwith the same amount of the Silicone Gum XIII prepared in Preparation10.

The thus obtained silicone rubber composition was cured in the samemanner as in Example 1 into a rubber sheet which has properties of: aspecific gravity d₂₅ of 1.43; hardness, JIS A, of 44; ultimateelongation of 345%; tensile strength of 107 kg/cm² ; and tear strengthof 18 kg/cm.

What is claimed is:
 1. A curable fluorine-substituted organopolysiloxanecomposition which comprises, in mixture(A) a polyorganosiloxane having aviscosity of at least 400 centipoise at 25° C. and a molecular structurerepresented by the general formula

    .sup.1 --O--(--SiR.sub.2 --O--).sub.n --(--SiRR.sub.f --O--).sub.m --X.sup.2,

or

    [X.sup.1 --O--(--SiR.sub.2 i--O--).sub.n --(--SiRR.sub.f --O--).sub.m --].sub.p SiR.sub.4-p,

in which R is a monovalent hydrocarbon group selected from the classconsisting of methyl, vinyl and phenyl groups, R_(f) is a fluorinatedalkyl group having 3 to 10 carbon atoms, n is zero or a positiveinteger, m is a positive integer with the proviso that n:(m+n) is in therange from 0.005 to 1, p is 2, 3 or 4, X¹ is a vinyl dimethylsiloxy-substituted silyl group represented by the general formula

    Vi-SiMe.sub.2 --(--O--SiMe.sub.2 --).sub.r --,

in which Vi is a vinyl group, Me is a methyl group and r is a positiveinteger in the range from 1 to 30, and X² is a hydrogen atom or X¹ ; and(B) an organic peroxide curing agent in an amount sufficient to cure thecomponent (A).
 2. The curable fluorine-substituted organopolysiloxanecomposition as claimed in claim 1 which further comprises (C) a finelydivided silica filler having a specific surface area of at least 50 m²/g in an amount up to 100 parts by weight per 100 parts by weight of thecomponent (A).
 3. The curable fluorine-substituted organopolysiloxanecomposition as claimed in claim 1 wherein the amount of the organicperoxide as the curing agent is in the range from 0.1 to 10 parts byweight per 100 parts by weight of the component (A).
 4. The curablefluorine-substituted organopolysiloxane composition as claimed in claim1 wherein the group denoted by the symbol R_(f) is a3,3,3-trifluoropropyl group.
 5. The curable fluorine-substitutedorganopolysiloxane composition as claimed in claim 1 wherein the groupdenoted by the symbol R_(f) is a 2-(perfluorobutyl)ethyl group.
 6. Thecurable fluorine-substituted organopolysiloxane composition as claimedin claim 1 wherein the group denoted by the symbol R_(f) is a2-(perfluorooctyl)ethyl group.
 7. The curable fluorine-substitutedorganopolysiloxane composition as claimed in claim 1 wherein X² is X¹.8. The curable fluorine-substituted organopolysiloxane composition asclaimed in claim 1 wherein X² is a hydrogen atom.